Combination Foundation

[egc]

We have a project where we have oversight reponsibility. The allowable contact pressure is 2000 psf. Since some of the equipment has dynamic loads (a debarker for instance), the design engineer has piling under large mat. In this case (95)-60k piling under a 32' wide a 116'6 long 2'3" thk mat. The piling are Auger cast in-place 16" dia to 40' BG. The total load super-imposed on the piling is about 4200k (including the mat weight). The piling arrangement grid is 7'6 spacing except near the 4 piers where the spacing goes to 4'6.

My view of the piling efficency (66%) is that the layout needs to be revised. I also think that since the adjacent equipment is on shallow foundations that the design for this piece should utilize piling below the piers only and let the mat carry the other loading (small conveyors, shrouds at either end, and of course the mat weight).

Again, the soil at this area has heavy shallow foundations (mats).

Any comments? I've had other engineers speak both pro and con. I had one tell me that most A&Es put all loads on piling regardless as they are the much stiffer carrying elements then the soil. It just strikes me as wasteful.

 

[msucog]

is hard to say without knowing all the details about subsurface conditions and configuration of the mat, pilings, equipment, loads, etc etc. as well as expected performance, accepted level of risk of owner, and on and on.

i'd rely on the project geotech and discuss the hows/whys with them to see if there are other approaches that can be utilized. by the way, on a side note i seldom use the term "wasteful" as this can create unnecessary and unjustified headaches within a project team...it's either more/less conservative. i have thrown out an "overly conservative" on rare occassion when a lowball geotech got the job because they sold a no-brain approach to the exploration instead of actually engineering the solution (is much easier/cheaper to default to deep foundations instead of engineering say shallow foundations on a tough site for example).

 

[hokie66]

Design of footings always has components of engineering judgment involved. Do you think your judgment is better than the design engineer's?

I wouldn't want a footing part supported on piles and part supported on ground with only 2000 psf capacity.

 

[mudman54]

recent clyde baker lectures suggest that the piles may be used for ground improvement. in that case, a crushed stone mat is used to separate the piles from the mat. try to find out from the designer why the piles are needed for the dynamic loads anticipated on the mat or if they're just being conservative.

 

[VAD]

I may want to agree with you. Very often we can be guided with what has been built and is performing satisfactorily providing the adjacent conditions are the same and loads etc are similar. A piled raft foundation akin to what has been designed would utilize the piles to mitigate settlement to some extent. Mudman gives some sound ideas regarding the use of piles. Poulous has been flooging this type of design for soft ground areas. Hookie 66, I believe that the 200psf represents the contact pressude imposed by the foundation and is not necessarily the "capacity" of the soil. Msucog comments on subsurface conditions are important. Is the existing mat subjected to dynamic loads as well. Is the designer concerned about liquefaction. Are the soils and ground water conditions amenable to this effect. I doubt it but who knows,

Based on some of my experience I would not be surprised that this is a conservative approach. However, we must give the designer some credence. As mudman says, the best is to have some dialogue with the designer. I wish that structurals and geotechs can have this type of discussion. however, i presume from the practice that this is astructural engs design with some recommendations from the geotech but little or no cross discussion. This is part of today's on going practice - The silo effect.

Interseting topic.